Since its conception, the GSM system was designed with the (now old) GSM half-rate (HR) and full-rate (FR) codecs. This pair of codecs lacked behind the quality of the wireline telephony

– (32 kbps adaptive differential pulse code modulation (ADPCM))

These two codecs satisfied the initial expectations of the GSM system. However, the user demand for better speech quality, along with capacity gains enabling cost savings for network operators motivated the further development of the speech services and codecs

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FR codec

Information - User 1

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HR codec

Information - User 1

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The Enhanced Full Rate (EFR) Codec (1)

Enhanced Full Rate (EFR) was the first significant improvement in speech quality in the GSM system.

It was introduced in ETSI (European Telecommunication Standards Institute) specifications in 1996, as well as in IS-641 in the USA the same year.

EFR codec was jointly developed by Nokia and University of Sherbrooke.

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The Enhanced Full Rate (EFR) Codec (2)

In spite of the commercial success of EFR, the codec left room for improvements

– The performance of the codec in severe radio channel error conditions could have been better.

– In addition, GSM half-rate codec was not able to provide adequate speech quality

So, it was decided to continue the codec specification work with a new codec generation… the AMR

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GSMGSM

HRHR &&

FRFR

EFREFR

AMRAMR

GSMGSM

AMRAMR

WCDMAWCDMA

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The Search for a New Codec

In October 1997 a new programme was initiated by ETSI aimed at the standardisation of Adaptive Multi- Rate for GSM system.

A one year feasibility study was carried out before to validate the AMR concept.

The main target of the work was to develop a codec that provided a significant improvement in error robustness, as well as capacity, over Enhanced Full Rate (EFR).

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The History of AMR in ETSI

The actual AMR codec standardisation was carried out as a competitive selection process consisting of several phases.

– In February 1999, ETSI approved the AMR codec standard, which was based on the codec developed in collaboration between Ericsson, Nokia and Siemens.

– In April 1999, 3GPP adopted the AMR codec as the mandatory speech codec for 3G WCDMA system

– In June 1999, some parts of the AMR codec were finalised and included in the standard

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What is Adaptive Multi-Rate? (1)

The AMR is an efficient quality and capacity improvement functionality.

Basically, it consists of the introduction of an extremely efficient and adaptable speech codec, the AMR codec.

The AMR codec contains

– A set of fixed rate speech and channel codecs,

– Fast in-band signalling and

– Link adaptation.

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What is Adaptive Multi-Rate? (2)

The AMR operates in both the full rate (22.8 kbps) and the half rate (11.4 kbps) GSM channel modes

AMR has the ability to adapt to radio channel and traffic load conditions and select the optimum channel mode (HR or FR) and codec mode (bit rate trade-off between speech and channel coding) to deliver the best possible combination of speech quality and system capacity

Codec mode adaptation for AMR is based on received channel quality estimation.

In order to maximise the commonality with GSM, all channels use polynomials used for the previous GSM channels.

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Block Diagram of the AMR Codec System

MobileMobile StationStation (MS)(MS)

SpeechSpeech

inin

SpeechSpeech

OutOut

Multi-

MI

Multi-

MI

Rate

Speech

S

Rate

Speech

S

Encoder

Encoder

MR

MR

Mode

Link Adaptation

Request

Generator

MC

Downlink

Quality

Meas.

Multi-

Multi-

Rate

Rate

Speech

Channel

S

Decoder

Decoder

MI

MC

3GPP SYSTEMS

BaseBase TransceiverTransceiver StationStation (BTS)(BTS)

TranscoderTranscoder (TC)(TC)

Radio

Channel

(UPLINK)

SS == SpeechSpeech DataData

QIQI == QualityQuality IndicatorIndicator

MCMC == CodecCodec ModeMode CommandCommand

MRMR == CodecCodec ModeMode RequestRequest

MIMI == CodecCodec ModeMode IndicatorIndicator

MC,MC, MRMR && MIMI == (In-band(In-band

Signalling)Signalling)

MI

Multi-

MI

S

Rate

S

MR

Channel

Decoder

MR

DownLink

Mode

Uplink

Control

Quality

Meas.

Uplink

Mode

Link

QI

Control

Adaptation

Multi-

Rate

MC

Channel

Encoder

S

MI

Multi-

Rate

Speech

Decoder

MI

S

AbisAbis

MC

S

Radio

Channel

MC

(DOWNLINK)

S

MI

Multi-

Rate

Speech

AbisAbis

MI Encoder

SpeechSpeech

OutOut

SpeechSpeech

InIn

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Link Adaptation

Both the MS and the BTS perform channel estimation of the received signal. Based on channel quality measurements, a codec mode command (down-link) or codec mode request (up-link) is sent over the radio interface in in-band messages. The receiving end uses this information and selects the best codec mode for the prevailing channel conditions The codec used in the up-link may be different to the one used in the down-link, However, the channel mode (HR or FR) MUST BE THE SAME

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In-Band Signalling

An in-band signalling channel is defined for AMR that enables the MS and the BTS to exchange messages on applied or requested speech and channel CODEC modes.

The benefit of in-band method is that it does not require a separate signalling channel for the message transfer.

With in-band signalling, messages and indicators are sent together with the speech payload.

This makes Link Adaptation faster and leads to improvements in the system performance.

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Codec Selection - MS and BTS operation (1)

The BTS controls the UL and DL codec modes and channel modes. BTS commands the MS to apply a particular speech CODEC mode in the up-link,

The MS MUST obey the codec mode command.

– The MS can only request the BTS to apply a particular speech CODEC mode in the down-link

The BTS, on the other hand, may use complementing information, in addition to codec mode request, to determine the downlink codec mode.

– That is, the BTS has an option to override the MS's request.

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Codec Selection - MS and BTS operation (2)

Although only a set of up to 4 speech CODEC modes is used during a call, the MS must implement all the codec modes. However, the BTS can support any combination of codecs, based on the choice of the operator,

– Except 7.95 kbit/s on HR channel, and it has one default set for each channel mode (I am not sure if this only applies to NOKIA).

The default CODEC sets include also a default set of decision thresholds and hysteresis.

The initial CODEC mode and CODEC set with thresholds and hysteresis are transferred between network elements and MS by using the existing layer 3 signalling

These techniques reduce radio interference and extend battery lifetime.

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AMR Narrow Band - AMR-NB (1)

GSM has the HR, FR, EFR and AMR speech codecs As explained, AMR was standardised in GSM Rel. ‘98 and has also been adopted in the UTRAN Rel. 99 as mandatory speech codec The AMR Rel’98 is also referred to as AMR-NB (AMR- Narrow Band) to distinguish it from the new AMR-WB (AMR-Wide Band) The difference between AMR-NB and AMR-WB lies in the speech coding bandwidth.

– The bandwidth for AMR-WB varies between 50 and 7000 Hz whereas the bandwidth of AMR-NB varies between 150 and 3500 Hz.

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AMR Narrow Band - Speech Codecs

The full-rate codec is actually a combination of eight speech codecs. The six lowest ones are used by the half-rate codec.

Bits per

Speech

Bit Rate

Speech

Average Code Rate GMSK Channel

Codec

(Kbits/s)

Frame

8-PSK Channel

FR

HR

FR

HR

GSM FR

260

0.57

GSM HR

112

0.49

EFR

244

0.54

AMR NB

12.2

244

0.54

0.36

10.2

204

0.45

0.3

7.95

159

0.35

0.7

0.23

7.4

148

0.32

0.65

0.22

6.7

134

0.29

0.59

0.2

5.9

118

0.26

0.52

0.17

5.15

103

0.23

0.45

0.15

4.75

95

0.21

0.42

0.14

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AMR - Wide Band (AMR-WB) (1)

The AMR-WB codec is based on similar principles as those defined for AMR-NB

The major difference in the deployment scenarios is that for preserving the superior quality of the wide-band speech, no transcoders are allowed in the transmission path of the speech from one terminal to the other.

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AMR - Wide Band (AMR-WB) (2)

The AMR-WB has nine different codec rates ranging from 6.60 to 23.85 Kbps.

– In GSM/EDGE single timeslot configuration, all these codec rates can be supported in the 8-PSK speech channels, and the code rates from 6.6 to 19.85 Kbps can be supported in the GMSK speech channels.

The link adaptation mechanism is similar to the one defined for the AMR-NB and based on the in-band signalling bits that are transmitted along the speech frames.

AMR-WB has been selected by 3GPP as its recommended wide-band speech codec.

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AMR Wide-Band (AMR-WB) (3)

Speech codecs supported by AMR-WB

Bits per

Speech

Bit Rate

Speech

Average Code Rate GMSK Channel

Codec

(Kbits/s)

Frame

8-PSK Channel

FR

HR

FR

HR

AMR WB

23.85

477

0.35

0.7

23.05

461

0.34

0.67

19.85

397

0.87

0.29

0.58

18.25

365

0.8

0.27

0.53

15.85

317

0.7

0.23

0.46

14.25

285

0.63

0.21

0.42

12.65

253

0.55

0.18

0.37

8.85

177

0.39

0.13

0.26

6.6

132

0.29

0.1

0.19

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Contents

Principles of AMR

AMR - Ericsson’s Implementation

AMR - Nokia’s Implementation

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Coding Sets

In every call setup and handover two parameters are selected by this algorithm

– A Coding Set and

– A Channel Rate/Mode

A Coding Set (CS) is a group of up to four CODEC rates/modes that an MS or a BTS could use

– All of them using the same channel rate (FR or HR).

A Channel Rate/Mode is the speed (in Kbps) of the codec selected.

This set is called “ACTIVE CODING SET ”.

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Principle View of AMR

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Decision Thresholds

For each of the predefined CODEC sets, there is an associated set of “Decision Thresholds” that determine which CODEC rate should be used for a certain C/I.

This algorithm allows a CODEC rate to be changed every second speech frame.

This change can be done only to the closest higher or lower CODEC rate in the active coding set ( Fast CODEC mode adaptation.)

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Defined CODEC Rates for AMR

Different CODEC rates can be used on the uplink and downlink (the set of CODEC rates is the same).

The CODEC rate changes are not audible.

The active coding set is predefined.

All TRXs /TRUs in the same cell must also support AMR

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In-band Signalling (1)

Quality measurements on the incoming link to perform the CODEC rate adaptation are done by both MS & BTS receiving side. The Quality Indicator produced on the MS side is converted to a “Mode Request” that is sent to the BTS using “In-band signaling”.

Transcoder

B

T

S

1. Codec Mode Request

UplinkUplink

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In-band Signalling (2)

There (@BTS), a CODEC mode control entity selects the CODEC mode to be used on the downlink, based on the Mode Request sent by the MS.

The CODEC mode control entity also selects the CODEC mode to be used on the uplink, based on the Quality Indicator produced in the BTS. The selected uplink mode is sent in-band to the MS.

This mode indication is sent every second speech frame.

Transcoder

B

T

S

2. Codec Mode Command

CodecsCodecs toto bebe usedused

inin UplinkUplink && DownlinkDownlink

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AMR Functional Diagram

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CODEC Mode Adaptation

When AMR is chosen at call setup:

– A CODEC set is communicated to the MS and the BTS.

The selection of CODEC mode within the CODEC set is based on measurements of the up-link and down-link radio channel.

The CODEC modes in an Active CODEC Set are defined as follows:

Codec Mode

Decription

1

Codec Mode with the lowest bit rate

2

Codec Mode with the second lowest mode

3

Codec Mode with the third lowest mode

Codec Mode with the highest mode (if the

4

Active Coding Set includes four modes)

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AMR Threshold Definition

For each of the predefined CODEC sets (two for Full Rate channels) there is an associated set of “decision thresholds” that determine which CODEC mode should be used for a certain C/I.

A “decision thresholds” threshold (THR) is given as an absolute value between 0 and 63 which represent normalized C/I values as follows:

Threshold

Normalised C/I (dB)

0

0.0

1

0.5

2

1.0

…

62

31.0

63

31.5

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AMR Hysteresis Definition

To avoid continuous rapid changes between 2 CODEC modes a “hysteresis” is used for the decision thresholds.

The “hysteresis” values (HYST) are given as an absolute value between 0 and 15 and are coded as follows:

Hysteresis

Threshold (dB)

0

0.0

1

0.5

2

1.0

…

14

7.0

15

7.5

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AMR Thresholds & Hysteresis

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AMR Multi-Rate Configuration

At Call setup and handover, the CODEC set to be used is sent from the BSC to the MS and the BTS together with the “decision threshold” and “hysteresis” values.

There are two full rate CODEC sets pre-defined in the BSC.

– Full Rate Codec Set 1

– Full Rate Codec Set 2

These CODEC sets with their associated “decision threshold” and “hysteresis” values are hard-coded and can not be changed.

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Full Rate Codec Set 1

This CODEC set has been selected because extensive evaluations have shown that it provides a good overall speech quality during all radio conditions.

It allows the operator to take full advantage of the possibility to use AMR as a capacity booster in the network.

Full Rate Codec Set 1

CODEC

RATE

Codec_Mode_1

4.75

Kbps

Codec_Mode_2

5.90

Kbps

Codec_Mode_3

6.70

Kbps

Codec_Mode_4

10.20 Kbps

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FR Codec Set 1 - Thresholds and Hysteresis

Decision thresholds and Hysteresis values for full rate CODEC set 1

Threshold

Value

dB

Hysteresis

Value

dB

THR_1

12

6

HYST_1

2

1

THR_2

21

10.5

HYST_2

3

1.5

THR_3

21

10.5

HYST_3

3

1.5

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Full Rate Codec Set 2

This CODEC set provides basically the same overall speech quality as full rate CODEC set 1 but offers a slightly better speech quality under clean speech conditions.

Full Rate Codec Set 2

CODEC

RATE

Codec_Mode_1

4.75

Kbps

Codec_Mode_2

5.90

Kbps

Codec_Mode_3

7.95

Kbps

Codec_Mode_4

12.2

Kbps

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FR Codec Set 2 - Thresholds & Hysteresis

Decision thresholds and hysteresis values for full rate CODEC set 2

Threshold

Value

dB

Hysteresis

Value

dB

THR_1

12

6

HYST_1

2

1

THR_2

17

8.5

HYST_2

3

1.5

THR_3

25

12.5

HYST_3

3

1.5

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Initial CODEC Mode (ICM)

It is sent from the BSC to the BTS and MS at call setup and handover.

The ICM is determined by the following rule:

– If the ACS contains 1 CODEC mode, then this CODEC shall be the ICM.

– If the ACS contains 2 or 3 CODEC modes, then the ICM shall be the most robust CODEC mode in the set.

– If the ACS contains 4 CODEC modes, then the ICM shall be the second most robust mode.

This means that 5.9 kbps will be the ICM for both full rate CODEC sets. !!!!

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Ericsson AMR Parameters

Parameter Name

AMRSUPPORT

Default Value

0

Rec. Value

**

Range

0 - 4

Unit

0 = OFF

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1=Full rate CODEC set 1 3=Not Defined

2=Full rate CODEC set 2 4=Not Defined

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Contents

Principles of AMR

AMR - Ericsson’s Implementation

AMR - Nokia’s Implementation

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NOKIA’S AMR Solution (1)

AMR CODECs are supported by different NOKIA base station generations as follows:

– NOKIA MetroSite and UltraSite base stations have full AMR support however NOKIA UltraSite EDGE TRXs have AMR support in CX3.1 software release.

– AMR CODECs support in NOKIA BSC and TCSM:

– All the NOKIA BSCs have full AMR support. except 7.95 kbit/s on HR channel. NOKIA TCSM2/E has full AMR support.

– TC PCM pool type is needed for transcoder configuration in A-interface. At minimum the basic AMR type, which supports FR AMR and HR AMR (pool 23), is at least implemented. The other pool types support is still for further study.

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Nokia’s AMR Solution (2)

Sub-multiplexing on highway PCM is 8/16 kbit/s,

– If AMR FR (16 kbit/s) is used in Abis interface, the Ater interface rate is also 16 kbit/s.

The same CODEC MODE has to be used in all 12 frames (it can be the same than the one used in the previous SACCH frame interval

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26 Frame Multi-Frame

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Link Adaptation and Decision Thresholds (1)

For each CODEC mode set there is an associated set of decision thresholds for mapping the channel quality measurements to the Mode Commands/Requests.

The Power Control (PC) and the Handover (HO) algorithms are affected by the Rx quality indicator in the following way

– If the quality is below lower thresholds then PC (more power) or HO is triggered depending on the current threshold values. Otherwise if quality is above lower thresholds, nothing is done.

– Respectively, if the quality is above upper thresholds then PC (less power) is triggered. Otherwise if quality is below upper thresholds, nothing is done.

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Link Adaptation and Decision Thresholds (2)

The aforementioned cases are valid only in quality HOs.

– They are not valid in, for example, power budget HOs.

The BSC indicates to BTS about which LA mode is used.

– Fast LA is the default mode of BSS.

– The octet defining the LA mode used shall not be sent to MS by BTS.

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Initial CODEC Mode (ICM) (1)

The Initial Codec Mode is determined according to ETSI specifications (as described in section I).

Channel allocation depends on the parameter Initial AMR channel rate, which default value is Any Rate.

Any Rate means that the chosen channel rate is defined by taking into account the currently used information (Channel Type IE, resource situation on radio interface, circuit pool, current channel rate, handover parameters, etc.).

The other option is AMR FR, which means that full rate channel is allocated despite of the values of the currently used information.

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Initial CODEC Mode (ICM) (2)

If AMR FR cannot be allocated, then allocation is continued with the currently used information.

The Initial AMR channel rate parameter is valid in

– call set-up (except FACCH call set-up),

– internal inter cell handover and

– external handover.

The reason behind the Initial AMR channel rate parameter is that quality may not be sufficient for HR AMR call set-up (radio measurement is done on SDCCH).

AMR capable TRXs are allocated first for AMR speech call use and then for other speech calls.

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New Rx Qual Thresholds

New RxQual thresholds are specified for FR and HR AMR sets.

Default values for these new thresholds are set according to the default AMR CODEC sets.

Current Nx and Px values of RxQual thresholds are used.

– Nx is total number of samples and Px is a set of them

If an operator wants to replace or remove the most robust mode on the AMR set, the corresponding PC and HO RxQual thresholds have to be edited manually.

– This also applies to the least robust mode.

– Replacement or removal of a middle mode on a AMR set does not effect the new PC and HO thresholds.

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New Rx Qual HO Thresholds

New RxQual HO thresholds are specified for FR and HR AMR and they are taken into account when making intra-cell handovers between FR AMR and HR AMR:

– intra HO threshold Rx qual for AMR FR

– intra HO threshold Rx qual for AMR HR

Current Nx and Px values of RxQual thresholds are used.

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AMR and Common BCCH Control

Decisions on the need for packing AMR full rate (FR) calls to half rate (HR) AMR calls is based on the load situation of each individual BTS also in the segment environment.

FR AMR calls in a certain BTS that should be packed is decided based on the load of that BTS only.

The intra cell handovers that perform the actual packing of calls are implemented as BTS internal events.

– When an intra segment TCH handover is made in order to decrease the load of a BTS, the amount of the possible requests for AMR full rate call packing in the BTS is decreased in order to avoid unnecessary handovers.

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Packing of FR AMR Calls to HR AMR Calls

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Packing of FR AMR Calls to HR AMR Calls due to BTS Load (1)

Spontaneous packing of FR AMR calls to HR AMR calls is triggered when the cell load is high.

The parameter (actually a threshold) Lower limit for FR TCH indicates the number of free full rate resources

When the number of free FR resources reduces below the value of this parameter, the packing process starts.

Packing continues until the cell load is low enough.

That is, until the number of free full rate resources increases above the value of the parameter Upper Limit For FR TCH resources.

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Packing of FR AMR Calls to HR AMR Calls due to BTS Load (2)

When packing is necessary, the BSC makes a request to perform an intra-cell HO for N amount of calls.

– N is defined by the BSC and it follows the principle that the number of free full rate resources increases by one compared to the situation before the new channel allocation.

In order to support 2 generation BTSs in AMR environment, DADL/B is used to handover AMR calls to co-located AMR capable cells during call set-up phase.

Both intra and inter BSC DADL/B handovers are possible and preferably inside one frequency band as the failure probability is much higher with DADL/B handovers between bands.

If AMR call is the aim and there are no TCHs available in the accessed cell, then Directed Retry (DR) due to congestion, with or without queuing, is made.

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DADL/B (2)

If on the other hand there are TCHs available in the accessed cell and there are adjacent cells defined as DADL/B handover target cells with the parameter amr Dadlb Target Cell, then the DADL/B handover is applied.

Adjacent cells are not verified according to the MS capabilities (single band, dual band or tri-band), but they have to fulfill the current signal level requirements in order to be considered as a target cell for DADL/B handover.

If there are no DADL/B handover target cells defined, the TCH is allocated from the accessed cell and another speech codec than AMR is chosen.

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Default AMR CODEC Sets for FR and HR (1)

BTS-MML (Mobility Management Layer ??) is used to take care of entering, modifying and displaying the new multi-rate configuration, that is, AMR CODEC sets, for BTS.

This parameter is used to define whether the initial CODEC mode used by the mobile station is defined explicitly in the AMR CODEC mode set or it is implicitly derived by the mobile station from the amount of CODEC modes in the AMR CODEC mode set.

– If the AMR FR CODEC mode set parameter is defined as disabled, then this parameter is set as 0.

– If the parameter is defined as 0, then the AMR FR start mode parameter is set as 00.

Range

Definition

Initial CODEC mode is defined by the

0

implicit rule provided in GSM 05.09

Initial CODEC mode is defined by the

1

Start Mode field.

3GPP SYSTEMS

Advanced Technology Group

WFI PROPRIETARY

User Interface with Enhanced Speech CODECs (1)

Channel rate and speech CODEC change control in internal inter-cell and external handovers with the parameter tchRateInternalHo.

– The default is that the best cell offering the call serving type of TCH and speech CODEC is preferred to be primarily allocated during the speech connection.

TCH allocation optimisation is based on the cell load.

– It is based on intra-cell HOs between AMR channel rates.

– It is handled with the parameter btsLoadDepTCHRate.

– Defaults for the lower and upper limit are set so that TCH allocation optimisation is off.

3GPP SYSTEMS

Advanced Technology Group

WFI PROPRIETARY

User Interface with Enhanced Speech CODECs (2)

The preference between the currently used AMR CODEC set and the one defined for the target BTS during internal and external handovers is handled with parameter amrConfInHandovers.

– The default is the CODEC set currently used.

The definition of the initial channel rate, Any Rate or AMR FR, in call set-up, internal inter-cell HO and external HO is handled with parameter initAmrChannelRate.

– The default is the Any Rate, meaning that there are no extra requirements by this parameter

– The chosen channel rate is defined by taking into account the MSC input and other parameters effecting to channel allocation.

3GPP SYSTEMS

Advanced Technology Group

WFI PROPRIETARY

User Interface with Enhanced Speech CODECS (3)

The AMR CODEC mode set can be downgraded or upgraded after internal inter-cell HOs.

The source side downgrade is used to align the source side CODEC set with the one chosen for the target side in order to utilise the uni-directional downlink connection for the target side.

The utilisation of uni-directional downlink connection decreases the amount of muting experienced in the speech path connection during internal inter-cell HO.

3GPP SYSTEMS

Advanced Technology Group

WFI PROPRIETARY

User Interface with Enhanced Speech CODECS (4)

The target side upgrade is made to utilise the original AMR CODEC set of the target BTS which set was not chosen for the target side because of target side alignment with the source side during internal inter-cell HO. The reason for this alignment is the same as in the downgrade case.

AMR Codec Mode set upgrades and downgrades are handled with parameter amrSetGradesEnabl.

The default is No, meaning that no source or target set alignment is performed.

intra HO threshold Rx qual AMR FR defines the threshold level of the signal quality downlink and uplink measurements for triggering an intra-cell handover process for an AMR FR call in order to switch it to an AMR HR call.